When sonar surveys spotted a vast pile of rubble in the Columbia River below Bonneville Dam late last winter, officials suddenly worried part of the dam structure was eroding into the river.

"Everybody said, 'Oh my gosh, we need to get divers out there right away,'" recalled Dennis Schwartz, a fisheries biologist with the U.S. Army Corps of Engineers, which oversees the dam.

What they found below the spillways in February was not a giant pile of rock at all, but a humongous pile of thousands upon thousands of sturgeon - some of them 14 feet long or longer - lounging together in frigid water at the bottom of the river.

"We call it the big sturgeon ball," Schwartz said.

The mountain of white sturgeon contained around 60,000 fish, according to a crude estimate by Michael Parsley, a research fisheries biologist with the U.S. Geological Survey's Columbia River Research Laboratory in Cook, Wash. He described that estimate as "probably conservative."

It was an aquatic phenomenon nobody had ever seen at such a monstrous scale, offering a startling glimpse into the life of the Columbia's largest and most ancient fish.

If the estimates are anywhere near correct, the congregation of sturgeon may represent 5 to 10 percent of all the white sturgeon in the lower Columbia River, Parsley said. The conclave apparently broke up in March as the Corps increased water releases through the dam to help salmon, Schwartz said.

An Army dive team called out to inspect the dam discovered the sturgeon when it sent down a remotely operated submersible to take a look at what everyone thought was debris.

On videos recorded by the underwater camera, the fish appear to be lounging, many on top of each other, some even upside down, at depths of 40 to 50 feet. Biologists considered whether the fish may be putting up an organized defense against sea lions showing up increased numbers to gobble salmon and sturgeon below the dam.

They discovered the sturgeon ball just as sea lions started to show up, but they also point out that sturgeon have been known to gather -- though in lower numbers -- in other places where there are no sea lions.

"The correlation (with sea lions) would probably be pretty weak," Schwartz said. "They all seemed to be just hanging out together."

Similar winter gatherings of sturgeon have been documented far up the Columbia in Canada, but not at the tremendous numbers seen below Bonneville, Parsley said. Biologists are not sure why the fish collect that way.

"We don't know whether that one aggregation is in response to sea lions being there, or if they do this every year," he said.

The fish may bunch up for safety as they conserve energy during the cool months of winter, he said. The water was about 34 to 35 degrees.

"They were just lollygagging -- definitely not expending energy," he said. "There must be some social or survival advantage to it, but I'm not sure what it is."

Sturgeon are ancient survivors that have changed little since the time of the dinosaurs. They can weigh more than 1,500 pounds and live to be well over 100 years old.

Biologists have heard reports of congregations of sturgeon at the surface of reservoirs and rivers, including shallow parts of the Columbia. But there aren't very good estimates on how many sturgeon may be involved in those gatherings, Parsley said. "Sturgeons in general seem to be relatively social."

He heard an account from a woman who said the river came alive with fish. She told him she couldn't help but think of sharks when she saw the giant fish slice across the water surface.

Teams working at the dam have reported some unusual sturgeon activity since sea lions began appearing in larger numbers at the dam each spring in recent years, eating both salmon and sturgeon. Last year, for instance, crews saw large numbers of sturgeon along the bank of Cascades Island, one of the islands just below the dam.

Biologists speculated that the fish might have been trying to avoid sea lions, said Robert Stansell, a U.S. Army Corps of Engineers biologist at the dam.

Sea lions -- mainly protected Steller sea lions -- were spotted gobbling more than 600 sturgeon this year, although the number dropped off later in the spring, he said. Hundreds of sturgeon also were found in the dam's fish ladder last winter, which was unusual, he said.

Parsley said sturgeon are so poorly studied that biologists don't know much about their behavior. Big gatherings of the fish in the depths of the river may be more common than anyone realizes.

"I firmly believe they do this elsewhere in the river," he said.

While salmon are well known, biologists have spent far less time mapping the populations and habits of sturgeon. But the fish are by far the largest and longest lived fish in the river system -- and one of the most mysterious.

"They're the woolly mammoth, the saber-tooth tiger or the lion of the Columbia River," Parsley said. "There's just still a lot to be learned about them."

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